In the event that a device is produced by using a semiconductor wafer such as a silicon wafer, there are many steps from a process of working the wafer to a process of forming a device. There is a step of heat treatment as one of these steps. The step of heat treatment is an important process that is performed for the purpose of formation of a defect-free layer in a surface layer of the wafer, gettering, crystallization, formation of an oxide film, impurity diffusion, and the like.
As a diffusion furnace used in such step of heat treatment such as oxidation or impurity diffusion (an apparatus for oxidation and diffusion), with a larger diameter of the wafer, there has been mainly used a vertical furnace for heat treatment in which the heat treatment is performed with many wafers being horizontally supported at predetermined intervals. When the wafers are subjected to the heat treatment by using the vertical furnace for heat treatment, there is used a vertical boat for heat treatment for setting many wafers (hereinafter, occasionally referred to as “a boat for heat treatment” or simply “a boat”).
FIG. 4 schematically shows a conventionally general vertical boat for heat treatment. In the vertical boat for heat treatment 101, a pair of plate members 103 (also referred to as coupling members, or a top plate and a bottom plate) are coupled to both ends of four columns 102 (rods). Many slits 105 are formed in each column 102 and a convex part between each of the slits 105 serves as a supporting part 106 for the wafer. When the wafer is subjected to the heat treatment, the peripheral part of the wafer W is placed on the supporting parts 106 formed at the same height in each column 102, as shown in a plane view in FIG. 5(A) and a front view in FIG. 5(B), and thereby the wafer W is horizontally supported.
FIG. 6 is a schematic view showing an example of the vertical furnace for heat treatment. In the boat for heat treatment 101 carried into the interior of a reaction chamber 222 of the vertical furnace for heat treatment 220, many wafers W are horizontally supported. In the heat treatment, the wafers W are heated with a heater 224 provided around the reaction chamber 222. During the heat treatment, a gas is introduced into the reaction chamber 222 through a gas feed pipe 226, flows from the upper side to the lower side, and is discharged outside from a gas exhaust pipe 228. The gas to be used is different according to a purpose of the heat treatment. However H2, N2, O2, Ar, and the like are mainly used. In case of impurity diffusion, these gasses are also used as a carrier gas for an impurity compound gas.
Various shapes of the supporting part 106 for the wafer are adopted in the vertical boat for heat treatment 101, and each of FIGS. 7(A) and (B) shows an example thereof. In (A), semicircular supporting parts 106′ are formed by providing concave slits 105′ (grooves) in a cylindrical column 102′. In (B), on the other hand, rectangular supporting parts 106″ are formed by providing concave slits 105″ in a wide rectangular column 102″ so as to support a portion of the wafer W closer to its center than that in (A). There are other supporting parts having slits with another shape such as an arc shape or a hook shape.
In the case of using the vertical boat for heat treatment, when the heat treatment is performed at a high temperature particularly for the purpose of oxidation or impurity diffusion and the like, internal stress is caused due to a wafer's own weight, or thermal stress is caused due to non-uniform temperature distribution in the wafer. When these stresses exceed a certain critical value, slip (slip dislocation), which is a crystal defect, is generated in the wafer. It has been known that since this critical value for the generation of the dislocation becomes rapidly smaller at a higher temperature, the slip dislocation is more easily generated with a higher temperature. Forming a device at a location where the slip dislocation has been generated causes junction leakage and the like, and thereby a yield of a device fabrication may be remarkably reduced.
For example, in the case of using a conventional boat with supporting parts 106′ or 106″ formed as shown in FIGS. 7(A) and (B), the slip is easy to be generated at a portion which comes in contact with the tip of each supporting part. This is because the wafer may be brought into point contact with the supporting part at the tip.
For example, in the case of the boat for heat treatment merely in which each of the supporting parts is CVD-coated with SiC, its surface has Ra (center line average roughness) of approximately 1 μm, that is very rough, and thereby, when the wafer is placed on the supporting part, the wafer is supported at a very small elevated portion (a local projection) by the point contact. It is considered that the internal stress due to the wafer's own weight is consequently increased locally, so that the slip is easy to be generated.
In order to prevent the generation of the slip as described above, measures are taken such that the tip of the supporting part is chamfered or the elevated portion of the supporting part for the wafer is removed by polishing its surface and the like.
However, the supporting part of the boat for heat treatment has a problem of being broken easily during chamfering or polishing processing with a machine and the like because it is thin and fragile. If a single supporting part is broken, the whole of the boat becomes a failure product. Accordingly, a complete mirror-polishing requires a manual operation and the like, but surface roughness of each supporting part tends to vary. In addition, the mirror-polishing of all supporting parts requires a lot of labor and the boat consequently becomes very expensive.
Moreover, establishment of the optimal shape of the supporting part with respect to the surface roughness, the chamfering of the tip and the like requires production of various boats for heat treatment having various surface roughness and various chamfered shapes and a lot of preliminary experiments. However, since the boat for heat treatment is expensive, the experiments with various boats for heat treatment costs a lot.
In order to solve these problems, Japanese Unexamined Patent publication (Kokai) No. 2004-241545 discloses a boat having auxiliary supporting members removably attached to the wafer supporting parts. According to the disclosure, in case of the described boat, since the auxiliary supporting member can be removably attached, the chamfering or polishing processing can be easily performed, at low cost, on a surface of a desired auxiliary supporting member where the wafer is to be placed, and the generation of the slip can be effectively suppressed by attaching the auxiliary supporting member subjected to the polishing processing or the like to the supporting part to place the wafer on it and by performing the heat treatment.
With regard to materials of the boat, quartz (SiO2), silicon carbide (SiC), silicon (Si) and the like are usually used, e.g. for a silicon wafer to prevent contamination of the wafer. For example, in heat treatment processing at a high temperature over 1000° C., the boat composed of SiC or Si, which has higher heat resistance than the boat composed of quartz (SiO2), is used. In particular, since the SiC boat can more reduce metal contamination generated during the heat treatment by CVD-coating it with SiC, it is used more often.
However, when the surface CVD-coated with SiC is subjected to, for example, mirror-polishing, a surface layer portion of a SiC film has a high Fe metal contamination concentration in some cases. In this case, when the wafer is placed to perform the heat treatment, Fe metal contamination transfer may occur. The metal contamination transfer can be prevented by forming an oxide film on the surface of the boat for heat treatment. For reasons of this, there has been used the auxiliary supporting member composed of SiC in which the oxide film is formed on its surface.
However, when the wafer is placed on the auxiliary supporting member composed of SiC in which the oxide film is formed and is subjected to the heat treatment under an argon atmosphere, an amount of the Fe metal contamination transfer is reduced, but a back surface of the silicon wafer is roughened by the oxide film in some cases.
On the other hand, in the event that the oxide film is not formed on the auxiliary supporting member, there is a problem such that Fe generated from a side face of the auxiliary supporting member contaminates the surface of the wafer placed in a next stage.